Experimental Charcoal Making
 
We decided to have a second go at traditional charcoal making at Brian’s experimental iron smelt in the National Heritage Park in Ferrycarrig, Co.Wexford. Once again our experiment was based on the early medieval charcoal production pit excavated by Ellen OCarroll and IAC Ltd in Russagh, Co. Offaly. This time round I had invaluable advice and help from Eoin Donnelly of Muintir na Coille who still makes charcoal on a seasonal basis in steel kilns. Many thanks to Eoin for all the help!!

What we did

The pit we dug was exactly the same as the last one and it measured  circa 1.4m x 0.91m x 0.51m and it was a little bit deeper than the excavated example (which measured 0.21m) so as to allow for the topsoil depth and truncation.

Saturday Afternoon

We dug and stacked the pit on Saturday afternoon (after strenuously weighing all our fuel wood). This time we decided to create two flues or chimneys at the sloping edge of the kiln. To create one flue we simply laid two lengths of timber parallel to each other, c.20cm apart, down the sloping side of the kiln leading from the top of the kiln right down to the base.

We packed the base of the kiln and the two flues with dry hazel kindling and straw.

Across the two flues (four lengths of timber) we stacked our oak wood along the long axis of the rectangular pit. On top of the oak wood we laid a layer of dry bracken. On the opposite side of the kiln directly across from the flues we placed two hollowed out lengths of timber (old wooden land drains) set upright in the kiln to act as smoke outlets for the kiln.
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The dug pit with the two flues on the sloping side and hazel kindling
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Lengths of oak timber stacked horizontally along the long axis of the pit

Sunday Morning


We fired the pit kiln early the next morning (Sunday) at 08:00. We lit two fires on the sloping edge of the kiln with hazel and oak kindling just above the two flues/ chimneys spaces so that the fire would then spread down through the kindling packed chimneys to the base of the pit. We got the fire going really well within the pit and then started to gradually cover the kiln by placing grass turfs (grass side down) over the layer of bracken and around our two upright chimneys.
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The charcoal production pit just after it was fired on the sloping edge
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View down one of the flues c.2 hours after kiln was fired
On the sloping edge of the kiln just prior to completely covering the pit with grass turfs and soil we inserted three metal pipes to act as air flows/ smoke outlets in the kiln. Of course back in early medieval times there was no such thing as steel pipes!! A substitute for these three metal pipes would be to simply use three very green branches and once the wood was stacked around these they could be pulled out in order to create a cylindrical space that could be used as an air flow/ smoke outlet. Likewise instead of the hollowed out upright timbers a simple gap (chimney) in the outer covering of the pit would suffice.
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Charcoal pit complete with air and smoke inlets/ outlets. A charcoal kiln such as this is known as an improved traditional charcoal pit kiln. A simple substitute for these pipe and post air and smoke inlets would very simply be pierced holes in the outer covering
When making charcoal it is very important to create an oxygen limited environment but it is even more important to get an air convection flowing within the kiln to aid carbonisation and this was done through the use of air flows/ smoke outlets.

After the initial firing quite a lot of noxious dirty yellow smoke (gases) and steam were released from the kiln and this eventually slowed to a steady release of steam and smoke from the kiln over the next few hours. This release of steam was an indicator that carbonisation was taking place within the kiln i.e. that moisture was being slowly driven off the oak timber by the heat of the kiln.

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Charcoal pit several hours after firing
Sunday Night

After 16 hours of burning we decided to shut the kiln down at midnight on the Sunday so that the charcoal would have enough time to cool down for extraction on the Monday. Ideally we should have left the kiln for another few hours. We could tell from the steam and smoke outlet and from the shrinkage/ collapse of the pit covering that one half of the pit had burned well while the other half had not burned so well. To combat this at around 21:00 (circa 3 hours before we shut the kiln down) we opened another air flow hole on the less active side of the kiln by simply punching a hole in the cover with a shovel handle.
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View down one of the air inlet pipes inside the charcoal kiln
RESULTS

Monday Lunch time

After allowing the kiln to cool down for 13 hours (it was a particularly cold night - minus 3 degrees celcius) at 13:00 we decided it was time to open the pit and extract our charcoal...if we did in fact have any charcoal!!

AND WE DID! WE MANAGED TO MAKE CHARCOAL!!!

We cleared off the soil and grass turf covering (as you can see in the time-lapse video) and extracted our charcoal yield. The least effective half of the kiln contained many brands (partially charred timbers) and almost fully charred timbers as well as charcoal throughout while the other half contained much charcoal. We sifted through the kiln separating our oak charcoal from charred bark and kindling. After allowing it to cool further we were satisfied that it would not reignite so we bagged the extracted charcoal. In parts of the kiln many timbers were thickly coated with a black shiny tar. In total we managed to collect 14kgs of pure oak charcoal (a charcoal: fuelwood ratio of 1:12) so it wasn’t the most efficient charcoal burn. But then again this was not a very efficient way of making charcoal! Owing to the fact that we shut the kiln down early and half the contents of the pit was composed of brands a pit of this nature would probably at best be able to produce an output of 30kgs of oak charcoal – roughly the amount used in Brian’s experimental smelt!
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View of the charcoal yield (including brands) within the pit after we opened the kiln and cleared away the soil and turf covering
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The charcoal pit after it was 'excavated' and cleaned out. Note the baked clay at the base of the slope and the two fire-scorched patches on the upper section of the slope - where the kiln was fired
Interestingly the fire and heat scorched patterns on our reconstructed charcoal pit were very similar to the scorch patterns on the excavated charcoal pit at Russagh, albeit the original excavated pit was much more fire reddened.  

ALL IN ALL OUR EXPERIMENT WAS A SUCCESS...WE MANAGED TO RECONSTRUCT AN EARLY MEDIEVAL CHARCOAL PRODUCTION PIT AND MAKE CHARCOAL IN IT THROUGH TRADITIONAL METHODS!


Niall Kenny
 


This is a time-lapse video of Niall Kenny and Eoin Donnelly's experimental charcoal production pit at Smelt 2010 being opened and emptied. The charcoal burn went well and a substantial amount of charcoal was made. The charcoal burn was featured in the Naked Archaeology Podcast which can be downloaded here or listened to online here.

More updates on the charcoal experiments will be put up asap and you can have a look at some video from the smelt here

Brian

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    This site gives information about charcoal production experiments being undertaken by Niall Kenny, an archaeologist from Ireland (in collaboration with Brian Dolan and SMELT 2010)